This invention relates to apparatus and a method for performing dual energy radiography to enable subtraction of x-ray images so that in the resulting difference image, anatomical information that is obscuring is cancelled and image information that is of interest is retained and, furthermore, enhanced.
One known x-ray image subtraction procedure is characterized as energy subtraction. Energy subtraction is based on the fact that, while the attenuation or absorption of x-radiation by a material depends on the energy or wavelength of the x-rays, this energy dependence is different for different materials. Suppose having different materials 1 and 2 of thicknesses t.sub.1 and t.sub.2, respectively, and measuring the thicknesses t.sub.1 and t.sub.2 is the objective. If x-rays of energy E.sub.1 are transmitted through the unknown object the transmitted intensity will be: EQU I.sub.1 =I.sub.01 exp (-t.sub.1 .mu..sub.11 -t.sub.2 .mu..sub.21) [Eq. 1a]
where I.sub.01 is the unattenuated intensity and .mu..sub.11 and .mu..sub.21 are the attenuation coefficients of the two materials at energy E.sub.1. If an analogous measurement is made at energy E.sub.2 the following applies: EQU I.sub.2 =I.sub.02 exp (-t.sub.1 .mu..sub.12 -t.sub.2 .mu..sub.22) [Eq. 1b]
where I.sub.02 and I.sub.2 are the incident and transmitted intensities, respectively, and .mu..sub.12 and .mu..sub.22 are the attenuation coefficients of materials 1 and 2, respectively, at energy E.sub.2.
Reorganizing the foregoing equations yields: ##EQU1##
Those equations can be solved for t.sub.1 and t.sub.2 : ##EQU2##
Hence, with an object having two material components that differ in the energy dependence of their x-ray attenuation (caused by a difference in atomic numbers), separate images of the two components can be formed. This is done by first producing an image at energy E.sub.1 and image at energy E.sub.2 as in equations 2a and 2b. These two images can be combined as shown in Eqs. 3a and 3b to produce the material selective images. The images formed using Eq. 3a would have only contributions from material 1 since material 2 is subtracted out. Conversely, the image formed using Eq. 3b has material 1 cancelled or subtracted out. In this way, separate images of the bones and soft tissues in a human chest, for example, can be produced using images at two x-ray energies.
The derivation presented above is, unfortunately, rigorously valid only if the x-ray beams are monoenergetic. Typically, only broad energy spectrum or polyenergetic x-ray beams are available. In this more complicated physical situation Eqs. 1 and 2 are no longer strictly valid due to the energy dependence of the attenuation coefficients. However, Eqs. 3 are still acceptable for some applications. If more accurate estimates of t.sub.1 and t.sub.2 are required, they are generally produced by higher order expansions, for example: ##EQU3## The coefficients .nu..sub.11 to .nu..sub.25 are usually arrived at using fitting methods.
While expressions like Eq. 3, that is, lineas subtraction will be used herein to demonstrate energy subtraction, it will be understood by those skilled in the art that higher order combinations such as Eq. 5 could also be used. Higher order combinations are obtained by further processing of the image data in electronic circuits that perform the mathematical operations.
In general, by properly combining the low and high average x-ray energy images as in Eq. 5, the contribution of materials of a certain average atomic number can be cancelled out or at least suppressed. The choice of the coefficients (.nu..sub.11 to .nu..sub.15, for example) determines which materials are cancelled. For a given pair of energy different images, many energy subtractions may be generated.
The process of combining the low and high energy image information will be referred to herein as energy subtraction and sometimes merely as subtraction. It should be kept in mind, however, that it is not a simple arithmetic difference.
In one known type of energy subtraction, an x-ray image of a region of interest in the body, is obtained with a nominally low kilovoltage (kV) applied to the x-ray tube so the beam projected through the body has an average spectral distribution within a band having low average energy. Before or after the low energy image is cbtained, another image is obtained with a comparatively higher kV applied to the x-ray tube and it has a higher average energy spectral band. Various kinds of image detectors are used to form the low and high energy images. Typically, the low and high energy images are obtained with an x-ray image intensifier tube whose output phosphor is viewed with a video camera. The analog video output signals from the camera are converted to digital picture elements (pixels) corresponding in value to the intensities in brightness of the same elements in the original x-ray images. The digitized images are usually stored in a full-frame memory. Customarily, digital pixel data representative of the low and high energy x-ray images are variously weighted so that when the data are subtracted the resulting difference image suppresses vlsualization of materials lying within a certain atomic number range. In this way soft tissue structures can be subtracted out when the image information of interest is an underlying or overlying blood vessel containing an iodinated x-ray contrast agent. In other applications, the distracting effects of bony structures may be subtracted out to allow better visualization of soft tissue masses such as in the lungs. The digitally expressed pixels composing the difference image are usually converted to analog video signals and used to drive a television monitor that displays the difference image. In these subtraction procedures, it is very important that the images at the different average x-ray energies be acquired as close together in time as possible so that no mis-registration artifacts are generated or appear in the subtracted image.
A typical system for subtracting x-ray images obtained with different x-ray energies is described in Georges et al., U.S. Pat. No. 4,355,331.
Presently available systems that do energy subtraction are generally complex and ordinarily it is not practical to adapt them to existing conventional diagnostic x-ray systems. Moreover, prior image subtraction systems require a specialized x-ray tube power supply that has the capability of switching the kilovoltage applied to the x-ray tube between low and high kV levels at a rapid rate in order to obtain the low average photon energy and high average photon energy x-ray beams in quick enough succcssion to obviate the ill effects of patient movement. An x-ray tube power supply and control for switching the x-ray tube between high and low energy output states is described in Daniels et al., U.S. Pat. No. 4,361,901. The two cited patents are assigned to the assignee of this application.
There is a need for a system that permits subtraction of x-ray images but which obviates the need for an expensive electronic image receptor and instead uses radiographic film images formed with different x-ray energies. Such a system could be adapted to existing diagnostic x-ray apparatus wherein single radiographic exposures are made with the use of a relatively broad x-ray spectrum and a cassette that contains the film and the intensifying screens. Generally, in ordinary radiography the light-proof cassette has one intensifying screen fixed in its body and another screen attached to the cover of the cassette. Film having a photographic emulsion on both sides is inserted between the screens before the cover of the cassette is closed. When the x-ray exposure is made, the intensifying screens luminesce so that the light from them exposes the film. The intensifying screen that is first penetrated by the x-ray beam that emerges from the body will absorb, typically, a higher fraction of the incident soft or low energy x-ray photons incident on the first screen and a somewhat smaller fraction of the higher energy x-ray photons. This is because lower energy x-rays are more easily stopped. The absorbed x-rays will cause the screen to produce light which will expose the film. While a larger fraction of the light will expose the film emulsion closest to the first or front screen, some cross-over of light to the back emulsion will occur. The x-rays that were not absorbed by the front screen will impinge on the second or rear screen. Because of the natural differential absorption of the front screen, the x-rays absorbed by the rear screen will have a higher average energy than those absorbed by the front screen. The x-rays absorbed by the rear screen will excite it to produce light and expose the film emulsion, primarily, the back emulsion in this case. Thus, in a sense, two coincident x-ray exposures are made on a single film wherein each of the intensifying screens and emulsions has been excited by x-radiation at different average energies. However, the energy information is lost because it has been distributed between the two film emulsions in an uncontrolled manner.
The scheme just described is meritorious insofar as obtaining perfect registration between the low and high energy images is concerned since the images are made during the same exposure.
However, since the dual energy images are superimposed on the film they cannot be read out independently with a transmission or reflected light beam and detector system. Reading out the images separately is necessary in order to permit weighting of at least one set of image data and the cancellation or subtraction of image information which only obscures the information of primary interest.
In Alvarez, et al., U.S. Pat. No. 4,029,963, Column 8, beginning at line 42, a scheme is proposed wherein a double emulsion film is exposed between two similar intensifying screens such that the low energy x-ray photons will interact with one screen primarily and the high energy photons, from which a substantial amount of the low energy photons have been filtered out, will interact with the other screen to zhereby produce images at slightly different energy levels on each of the emulsions. The patent states that the emulsions are separated from the film substrate and constitute transparencies through which a light beam may be projected in sequence to derive the individual different energy images by viewing the emulsions with a television camera. There is no explanation of how the emulsions can be separated from the film base without destroying them or, at least, losing registration between them.